Fix issue with yank/paste in vim when inside tmux on os x

There might be an issue when using yank/paste in tmux on other
platforms. By might, I mean probably.

vimrc

@@ -14,11 +14,14 @@ colorscheme wombat
 set fileformats=unix,dos,mac    " support all three, in this order
 set nostartofline               " leave my cursor where it was
 set history=1000
-set clipboard+=unnamed  " turns out I do like is sharing windows clipboard
 set autoread        " Set to auto read when a file is changed from the outside
 set autochdir       " Change current directory to be directory of current file
 set colorcolumn=80
 
+if $TMUX == ''
+    set clipboard+=unnamed
+endif
+
 set backspace=start,eol,indent
 set whichwrap+=<,>,[,]